The present invention relates to a magnetizing system and a superconducting magnet to be magnetized therewith.
As conventional art relating to a magnet for use of magnetizing is already known, for example, that having a bulk superconductor, as a target to be cooled by a refrigerator, with using a coil-type superconducting magnet therein.
This magnet for magnetizing is located at a central portion of the superconducting magnet of the coil-type superconducting magnet, a magnetic center of which is cooled down to a very low temperature, and this superconducting magnet is disposed within a heat insulating vacuum container. In case when cooling the bulk superconductor, down to the very low temperature by the refrigerator, the bulk superconductor is disposed within the heat insulating vacuum container, and an end of the bulk superconductor is thermally unified or integrated with a cooling stage of the refrigerator through a heat conductor, indirectly, and thereby building up a bulk superconducting magnet.
The method for magnetizing comprises the following steps (1) to (4):                (1) Generating a predetermined static magnetic field by running current from a magnetizing power source, after cooling the coil-type superconducting magnet for magnetization down to the very low temperature;        (2) Disposing the bulk superconductor of the bulk superconducting magnet before cooling at the position of the center of magnetic field within a bore of the coil-type superconducting magnet for magnetization at room temperature. Herein, fluxes for magnetizing penetrate through within the bulk superconductor;        (3) Turning the power source of the refrigerator for the bulk superconducting magnet “ON”, to cool the bulk superconductor down to the very low temperature, equal or lower than a temperature of obtaining the superconducting, and thereby brining the bulk superconductor into the superconducting condition within the static magnetic field; and        (4) Demagnetizing the coil-type superconducting magnet for magnetization. The bulk superconductor captures the magnetic fluxes penetrating therethrough, and when completing the magnetization, it generates a magnetic field. The bulk superconducting magnet is taken out from an inside of the bore at room temperature, and thereafter the refrigerator for the bulk superconducting magnet keeps the operation thereof.        
Herein, as it was explained in the (3) mentioned above, there is necessity for the refrigerator for the bulk superconducting magnet to be operated under the condition that the coil-type superconducting magnet for magnetization generates the magnetic field.
In general, such the refrigerator mentioned above has a compressor and an expander for compressing/expanding a helium gas therein, since it operates under a refrigerating cycle, having processes or steps for compressing/expanding the helium gas as a working medium thereof. One example of the refrigerator is a one-unit type with the compressor, directly connecting the compressor and the expander, and another example of the refrigerator is a split type of connecting both with tubes, each being separated from each other.
With the split type, since there are useless spaces within the tubes and there is generated a pressure loss when the gas flows within the tubes, with a cooling efficiency thereof is lower than that of the one-unit type with the compressor. Because of lowering of the cooling efficiency and an increase of consumption of electric power, it is not a good policy to apply the split type from a viewpoint of energy saving. Then, explanation will be given hereinafter, on the case of applying the one-unit type with the compressor therein.
Since motor of the compressor, illustrated in FIG. 4, uses magnetic materials, such as, magnetic steel and a permanent magnet, for example, motor cannot be operated within a space of high magnetic field. In general, motor must be operated within a space of low magnetic field, i.e., equal or lower than 0.1 Tesla. On the other hand, it is necessary to generate a very high magnetic field, such as, 5 Tesla to 10 Tesla, for magnetizing a high magnetic field, at a central portion of the coil-type superconducting magnet for magnetization by means of the bulk superconducting magnet. For this reason, within the space near to an end of the coil-type superconducting magnet, to be disposed the compressor therein, there are leakage fluxes of several Tesla; therefore, it is impossible to dispose the above-mentioned compressor. The space where the compressor could be disposed, i.e., being equal or lower than 0.1 Tesla in the magnetic field, is at the position, separating by 0.4 m to 0.7 m from the end of the magnet. Also, because the magnet is disposed within a vacuum heat-shielding space, the distance between the center of magnetic field of the coil-type superconducting magnet and an end of a vacuum container is about 0.3 m. This is because of the following reasons:
A superconducting coil is built up through winding up a superconductive wire or cable by a large number of times, for generating the high magnetic field, and herein, for the purpose of increasing the stability on cooling of the superconducting coil under a very low temperature with a thermal capacity of metal, the superconductive cable is wound around a core of a cold accumulating body, for example made of copper, by the large number thereof, and therefore the weight of the magnet is heavy. A heat-shielding support body comes to be long, for supporting that weight by that heat-shielding support body within the vacuum space and for preventing heat from invading therein from the portion of room temperature, and therefore the distance between the superconducting coil and the end of the container for vacuum heat-shielding becomes too large. Accordingly, the distance between the compressor portion of the refrigerator and the bulk superconductor is about 0.7 m when the magnetizing static magnetic field is 5 Tesla, and is about 1.0 m when the magnetizing static magnetic field is 10 Tesla.    [Patent Document 1] Japanese Patent Laying-Open No. Hei 10-11672 (1998).